Increasing processing speeds of devices such as graphics processors, hard disks, network cards, and other high speed I/O devices have created a need for an increased bandwidth for communicating between devices. One way to increase bandwidth between the bridge circuit and the I/O device is to use a differential communication link such as PCI Express™, HyperTransport™, SATA, USB, and other suitable differential communication links. Such interfaces are a flexible, hybrid serial-parallel interface format that uses multiple differential communication links often referred to as lanes. Each link includes transmit lanes to transmit information and receive lanes to receive information.
During certain modes of operation such as during a non-transmission mode, a low power mode, and/or other suitable modes of operation, the transmit lanes are clamped (e.g., coupled) to the same voltage (e.g., common mode voltage). As such, it is necessary for a receiver associated with the receive lanes to detect this mode of operation by sensing a differential voltage swing of the differential communication link. For example, the receiver can detect that the transmit lines are clamped when the voltage swing is less than a low threshold and that the transmit lines are not clamped when the differential voltage swing is greater than a high threshold. However, the low and high thresholds are typically at small signal levels, which can make it difficult for the receiver to detect due to, among other things, physical variations in the receiver (e.g., caused during the manufacturing process).
Known receivers typically use an idle (or squelch) detection circuit to detect the voltage swing. For example, one known idle (or squelch) detection circuit typically includes a rectifier circuit 100, as shown in FIG. 1, to rectify the voltage swing thereby aiding in detection of the voltage swing. As shown, the rectifier circuit 100 is coupled to a first power source 102 and a second power source 104 (e.g., ground). The rectifier circuit 100 includes a current source circuit 106, a first transistor 108, and a second transistor 110 configured as shown. The first transistor 108 is responsive to a first voltage 112 (e.g., in+) of the voltage swing. The second transistor 110 is responsive to a second voltage 114 (e.g., in−) of the voltage swing. In response to the first voltage 112 and the second voltage 114, the rectifier circuit 100 provides a rectified voltage 116 via output terminal 118. When the rectified voltage 116 has a deviation from the common mode voltage that is less than the low threshold, the receiver detects that the transmit lines are clamped. In addition, when the rectified voltage 116 has a deviation from the common mode voltage that is greater than the high threshold, the receiver detects that the transmit lines are not clamped. Although, the rectifier circuit 100 works, it is rather inefficient at detecting small voltage swings that are used in many high speed communication links, which is undesirable.
Accordingly, a need exists for a circuit that is capable of efficiently detecting whether the transmit lanes are clamped to the same voltage while overcoming the aforementioned disadvantages of conventional circuits.